A halocarbon is an organic chemical molecule composed of at least one carbon atom bound covalently with one or more halocarbon atoms. Halocarbons have many uses and are used in several industries as solvents, pesticides, refrigerants, fire-resistant oils, ingredients of elastomers, adhesives and sealants, electrically insulating coatings, plastics and anaesthetics. An alternative term for halocarbons is “halocarbonated fluorocarbons”.
Examples of halocarbons which are used as anaesthetic agents typically include desflurane, isoflurane, sevoflurane, halothane and enflurane. These anaesthetics may be referred to as volatile anaesthetic agents because they are liquid at room temperature but evaporate easily to produce a vapour for inhalation by a patient to induce anaesthesia. These agents are administered to patients using the breathing circuit of an anaesthetic machine, also known as a Boyle's machine. A schematic diagram of part of an anaesthetic machine including its breathing circuit 2 is described below with reference to FIG. 1. The primary function of the anaesthetic machine is to mix oxygen with volatile anaesthetic agent, at a clinician-specified concentration, for delivery to the patient via the breathing circuit 2.
The anaesthetic machine and breathing circuit 2 comprises a network of piped gas for inhalation by a patient (not shown). Air, oxygen (O2) and nitrous oxide (N2O) are supplied respectively to the back bar 15 from an air pipe 3 or an air cylinder pipe 5, an oxygen pipe 7 or an oxygen cylinder pipe 9 and a nitrous oxide pipe 11 or a nitrous oxide cylinder pipe 13. Each gas pipe 3, 7, 11 supplies gas at 4 bar. Air and oxygen are supplied by cylinder pipes 5, 9, at 137 bar. Nitrous oxide is supplied by cylinder pipe 13 at 44 bar. To reduce the pressure of the gases supplied by the cylinder pipes 5, 9, 13 to match the pressure of the gases supplied by the gas pipe 3, 7, 11 each cylinder pipe 5, 9, 13 comprises a pressure reducing valve (PRV) 17 which reduces the pressure of gases supplied by the cylinder pipes 5, 9, 13 to 4 bar.
Each of the air, oxygen and nitrous oxide is delivered separately to a respective variable flow valve 19, which allows an anaesthetist to mix the air, oxygen and nitrous oxide as required. Each variable flow valve 19 further reduces the pressure of the gases to just over 1 bar. FIG. 1 shows the gases are delivered to the back bar 15, from left to right, via an air back bar pipe 18, an oxygen back bar pipe 20 and a nitrous oxide back bar pipe 22. It will be immediately apparent to the skilled person that the back bar pipes 18, 20, 22 may be arranged differently. For example, the back bar pipes 18, 20, 22 may be arranged from left to right in FIG. 1 in the following order: the nitrous oxide back bar pipe 22; the oxygen back bar pipe 20; and the air back bar pipe 18.
The back bar 15 comprises a vaporiser 10 and a pressure relief valve 16. The vaporiser 10 contains a vaporisation chamber 21 in which the agent 12 is housed. The vaporisation chamber 21 is arranged so that the agent 12 evaporates to form vapour 14 at the saturated vapour pressure of the agent 12. For example, if the saturated vapour pressure is at too high a concentration to deliver agent 12 to the patient, a variable bypass valve 23 allows the anaesthetist to control the fraction of gases supplied from the back bar 15 that pass through the vaporiser 10. Accordingly, the output concentration of volatile agent 12 within the gas flow leaving the back bar 15 is controlled.
The patient inhales gases via a face mask 4 which fits over and forms a seal around the patient's nose and mouth. The face mask 4 is connected to an inspiratory tube 6 which supplies gases containing an anaesthetic agent 12, and an expiratory tube 8 through which exhaled and unused gases and agent 12 are transported away from the patient. The inspiratory tube 6 and expiratory tube 8 are typically corrugated hoses.
The inspiratory tube 6 comprises a unidirectional inspiratory valve 25 which opens upon inhalation by the patient. When the unidirectional inspiratory valve 25 is in an open state, gas flows through the back bar 15, through the vaporisation chamber 10 where it mixes with vapour 14 from the agent 12. The gas mixed with agent vapour 14 is inhaled by the patient. In use, the breathing circuit 2 dispenses an accurate and continuous supply of anaesthetic agent mixed with oxygen/air/nitrous oxide (N2O) at a specific concentration to the patient at a safe pressure and flow rate.
The expiratory tube 8 is connected to an expiratory pipe 24 to which is connected a unidirectional expiratory valve 26 through which exhaled and unused gases pass when the unidirectional expiratory valve 26 is open. Gas that passes through the unidirectional expiratory valve 26 flows into a breathing bag 28. An exhaust pipe 30 leads from the breathing bag 28 to a variable pressure-relief valve 32.
A carbon dioxide (CO2) absorber canister 34 is connected to the expiratory pipe 24 and the inspiratory pipe 15 and arranged to allow gases to flow through the absorber canister 34 from the expiratory pipe 24 to the inspiratory pipe 6. The absorber canister 34 contains soda lime 36 which absorbs carbon dioxide from the gas that flows through the canister 34.
The configuration of the breathing circuit 2 illustrated in FIG. 1 is shown during inhalation of the gas/agent mixture by the patient. The movement of inhaled gases is shown by the solid arrows and the movement of exhaled gases is shown using dashed arrows.
Inhalation by the patient causes the expiratory valve 26 to close and the inspiratory valve 25 to open. This allows recirculated gas to flow from the breathing bag 28, through the absorption canister 34 which absorbs CO2 in the gas, and into the inspiratory pipe 6. The gas passes through the vaporisation chamber 10 where it mixes with the agent vapour 14. The resultant gas/agent mixture is administered to the patient via the unidirectional inspiratory valve 25 and inspiratory limb 6 of the breathing circuit 2 and the breathing mask 4. The patient breathes the gas/agent mixture into their lungs which dissolve some of the agent vapour 14 into the patient's blood. This leads to a reversible state of anaesthesia.
Upon exhalation by the patient, the expiratory valve 26 opens and the inspiratory valve 25 closes. The gases exhaled by the patient, including the portion of the agent vapour 14 that is not absorbed by the patient, flow back into the breathing circuit 2 via the expiratory tube 8. The exhaled gases flow into the breathing bag 28 and excess waste gas 38 is vented via the pressure-relief valve 32. A waste pipe 40 guides the vented waste gas 38 from the breathing circuit 2.
The vented waste gas 38 will contain at least trace amounts of unused anaesthetic agent vapour 14. Even trace amounts of anaesthetic in the air in a medical environment will have an effect on medical staff, continued exposure to which will cause adverse health conditions, such as headache, increased incidence of spontaneous abortion, congenital anomalies in babies and haematological malignancy. Accordingly, governmental agencies have set limits on the level of volatile anaesthetic agent that hospital staff may be exposed to. In the USA the level of volatile anaesthetic agent in the air of an operating theatre should not exceed 2 parts per million (ppm), and the level of N2O should not exceed 25 ppm. The limit set for volatile agent in the UK is 50 ppm, and for N2O the limit is set at 100 ppm.
In order to ensure that the environment within operating theatres and other medical environments stay within the above limits, the waste gas 38 which contains volatile anaesthetic agent vapour 14 is prevented from entering the atmosphere of medical environments.
To prevent the release of anaesthetic gases into the atmosphere of an operating theatre, in most developed countries, the waste gas 38 is “scavenged”. In hospitals and large veterinary practices, operating theatre suites are provided with a negative pressure circuit. The negative pressure circuit is connected to the exhaust pipe 40 of the anaesthetic machine. The negative pressure circuit extracts the waste gas 38 to the atmosphere via an output pipe at the top of the building. Anaesthetic users of smaller practices extract waste gas 38 from the exhaust pipe 40 using the circuit pressure following the variable pressure release valve 32, which is at a pressure lower than the breathing circuit, to pass waste gases 38 from the exhaust pipe 40 through activated charcoal canisters. Such charcoal canisters are typically able to absorb twelve hours of waste gas 38. However, a problem with charcoal canisters is that once they have been used they cannot be recycled and must be disposed of, which is costly. Furthermore, unused volatile agent captured by the activated charcoal canisters may be slowly released after disposal.
Volatile anaesthetic agents are halogenated fluorocarbons, and therefore their release directly into the atmosphere is particularly undesirable. Halocarbons containing bromine and chlorine groups, collectively referred to as chloroflouorocarbons (CFCs), exert a damaging effect on the ozone layer. Indeed, the release of CFCs from any industry is damaging to the ozone layer. In the stratosphere, light at higher wavelength breaks down the C—Cl/Br bond of CFCs which releases highly reactive free radical groups that break down ozone (O3), depleting the earth's UV protective barrier. Isoflurane and halothane are both CFCs. Each agent has a different reactivity due to the amount of free radical each agent releases, and the ease with which the carbon-halide group is broken. Halothane is the most reactive, due to the relative ease with which the Br group may be removed from the molecule, followed by isoflurane. Nitrous oxide (N2O) also has some ozone depleting potential.
In addition, N2O and all agents, including sevoflurane and desflurane, are potent greenhouse gases due to their ability to absorb infrared light. Desflurane is the most potent due to its long atmospheric half-life. One kilo of Desflurane is equivalent to approximately 2000-3500 kg of CO2.
The use of CFCs was curbed by the Montreal agreement in 1987 (and subsequent amendments). As a result, the use of CFCs in refrigeration and aerosols was banned and all CFC use not deemed ‘essential’ was monitored. Medical uses of CFCs are deemed ‘essential’ and are therefore unmonitored. With the banning of the use of CFCs in refrigeration and aerosols, the proportion of halocarbons released into the atmosphere due to medical use has increased and is likely to increase further. Currently, forty million anaesthetics are delivered per year in the US, and five million are delivered per year in the UK. The majority of these anaesthetics are delivered under the influence of volatile agents. In addition, it is estimated that medical use of N2O contributes 3% of US N2O emissions.
An alternative way to capture the agent vapour 14 from the waste gas 38 of the breathing circuit 2 is to subject the waste gas 38 to extreme cold using liquid oxygen. Halocarbons will crystallise at around −118°. However, due to safety issues surrounding the use of liquid oxygen and the practicalities of removing and separating crystalline volatile agents from super-cold oxygen pipework, this is not a viable option for most medical establishments.
Another prior art system to capture volatile anaesthetic agent from the waste gas 38 is to pass the waste gas 38 over silicon dioxide (SiO2), also known as “silica” for extraction by steam. An example of this type of prior art system is described in International Patent Application Publication No. WO 2011/026230 A1.
Similarly to the charcoal method described above, the waste gas 38 is captured from the exhaust pipe 40 and passed through canisters that contain granular SiO2 to which the agent 12 binds. Once the SiO2 is saturated with agent 12, the SiO2 canisters are removed for processing. During processing the SiO2 is subjected to a steam purge gas at high pressure and high temperature to separate the agent 12 from the SiO2. Collected anaesthetic agent must be purified to remove water and then separated by fractional distillation.